The unit of the photovoltaic system which is referred to here as an inverter comprises at least one DC/AC converter which inverts the direct currents generated by the photovoltaic modules into an alternating current. Typically, the inverter comprises, in addition to the DC/AC converter, busbars to which the individual collector lines are connected directly or via DC/DC converters. A plurality of DC/AC converters can also be connected to these busbars, for example one DC/AC converter for each phase of the alternating current. Switches for connecting the individual current collector circuits and/or for connecting the DC/AC converter(s) to an output-side AC power grid and the like are frequently also present.
The current collector circuits of the photovoltaic system are also referred to as combiners or combiner boxes. The collector lines which run from them to the inverter are frequently referred to as homeruns. A small or even large number of strings of photovoltaic modules can be connected in parallel in a current collector circuit. Fuses and/or diodes for protecting against reverse currents, and sometimes also switches for connecting the individual strings are frequently also provided in the current collector circuits.
Occasionally the strings which are connected in parallel by a current collector circuit are individually also referred to as substrings, and only the totality of the substrings which are connected in parallel via a current collector circuit is then referred to as a string.
In the event of no DC/DC converter being connected between the strings and the DC/AC converter of the inverter, the voltage which is present between the string lines in the photovoltaic system is precisely as high as the voltage which is present between the collector lines and also as high as the voltage present between the busbars of the inverter. However, the strength of the current increases as a result of the parallel connections in the current collector circuits and the inverter. The constant voltage typically lies in the range of several hundred to a few thousand volts, with relatively high voltages being associated with the advantage that relatively low current strengths and therefore also relatively low line cross sections are sufficient to conduct equal electrical power levels.
Arcs can occur owing to insulation faults of the insulation between the string lines of each string, between the collector lines of each pair of collector lines and between the busbars of the inverter as well as between each of these lines and busbars with respect to ground or components of the photovoltaic system which are at ground potential. The arcs which occur between the string lines, the collector lines or the busbars of the inverter or between one of these lines and ground are usually referred to as parallel arcs. In addition to the parallel arcs, an arc can also occur within one of the specific lines or at the electrical connection thereof to another line of the same electrical potential, and said arc is then referred to as a series arc. The cause of a series arc can be, for example, locally limited damage to the line or an increased contact resistance at an electrical connection to the line. A defective, e.g. incorrectly opening, switch can also bring about an arc across its contacts. Both types of arcs—i.e. both parallel and series arcs—constitute a hazard potential in addition to the insulation faults themselves. Therefore, burning arcs can, for example, increase existing insulation faults and also cause other wide-ranging damage. This also includes causing a fire of the photovoltaic system. It is therefore important to detect arcs which occur in a photovoltaic system early, in order to be able to extinguish them and eliminate their causes.
In a known method and detection device, measured values of electrical variables are acquired at at least one of the string lines per string in the current collector circuits, and the measured values of the electrical variables are analyzed for the presence of signs of an arc in the photovoltaic system and arc signals which indicate the presence of an arc in the photovoltaic system are generated if the present signs of the arc meet predefined criteria. An arc fault circuit interrupter (AFCI) is activated with the arc signal. The electrical variables whose measured values are acquired for the arc detection are, for example, the DC voltage between the string lines, the DC voltage of the individual string lines with respect to ground potential, the DC current through the string lines or the high-frequency electrical interference spectrum which is emitted by an arc via the string lines. The acquisition of the measured values of the electrical variables frequently takes place here on a side facing the strings of a termination capacitor, provided in each current collector circuit, which makes available a low impedance for high-frequency modulations of the current, caused by the arc, through the affected string lines.
The known method and the known device for detecting arcs in a photovoltaic system are unsuitable for acquiring arcs which occur in or between the collector lines or busbars of the inverter or between the collector lines or busbars of the inverter and ground, since they do not have a significant effect on the electrical variables measured at the string lines, in particular if they occur on a side of the respective termination capacitor facing the inverter. However, shifting the measurement of the electrical variables toward the inverter would not overcome this problem, since there not only the voltage jumps and current jumps caused by an arc in the region of the string lines can remain very small compared to the rated voltage and the rated current, but also the voltage jumps and current jumps caused by an arc in the region of the collector lines which is further away from the inverter can remain very small compared to said rated voltage and rated current. When detecting arcs on the basis of the high-frequency electrical interference spectrum emitted by them, there is the need here to disconnect the high-frequency signal from the frequently very high direct currents. Decoupling means which are suitable for this, in the form of reactors and transformers quickly become disproportionally large and expensive in the case of large DC currents. In addition, such decoupling means cause undesired electrical losses. In addition, even when suitable decoupling means in the form of reactors and transmitters can still be implemented with an acceptable degree of expenditure for the DC currents which flow in the collector lines, the high-frequency signal, to be detected, of the electrical interference spectrum on the side of the termination capacitor facing the inverter is strongly damped owing to the generally high inductance of the collector lines. The detection of a signal which is damped in such a way is possible only with difficulty owing to the low signal-to-noise ratio, and the information provided by downstream evaluation electronics which are complex in terms of their configuration is reliable only to a limited degree.
DE 196 17 243 A1 discloses a method and a device for measuring the distance of a cable fault at which an arc is burning from a switching or measuring station in a medium-voltage power grid. Here, the transit times of the electrical and/or acoustic signals, generated by the arc, in the cable are measured in the cable sections on both sides of the arc and the distances of the source of the fault from the ends of the cable section are calculated therefrom. This means that the signals which are generated by the arc itself are utilized. This involves both electrical pulses and structure-borne noise signals with a purely arbitrary signal profile. The profile of the signals generated by the arc is measured with a sensor, and after the conversion with a high-speed analog/digital converter, is stored as a digital signal in a memory. By filtering with a Fast Fourier Transformation and a correlation analysis it is possible to determine the exact time of the arrival of the signal which is emitted by the arc in the direction of the sensor. In order to acquire the structure-borne noise as an indication for an arc, a piezoceramic sound transducer is used.
EP 1 623 240 B1 also discloses that partial-discharge activities within an insulation of high-voltage conductors can also be detected by means of different methods, including chemical, acoustic and electrical methods. There is specific teaching to use sensors for the electrical detection of partial discharges electrically said sensors being embodied as capacitive, inductive or longitudinal voltage sensors.
DE 10 2010 026 815 A1 discloses a method and a device for determining the location of faults in cables which serve, in particular, for the transmission and distribution of electrical energy and are laid in the ground. An acoustic signal which is generated at a location of a fault in the cable, and is, in particular, a flashover noise or discharge noise triggered by an electrical pulse, is utilized. In order in this context to reduce the influence of superimposed interference noise which makes identification of the acoustic signal and therefore also determination of the location of the fault more difficult, a reception signal, which contains the acoustic signal generated at the location of the fault, is compared with stored signal patterns or features or feature sets of acoustic signals generated at a location of a fault, in order to identify the signal in the case of correspondence or a predetermined similarity with the signal patterns or features or feature sets.